CONTEXT: Since 1994, the US Public Health Service (PHS) has recommended routine, voluntary prenatal human immunodeficiency virus (HIV) testing and zidovudine therapy to reduce perinatal HIV transmission.OBJECTIVE: To describe trends in incidence of perinatal AIDS and factors contributing to these trends, particularly the effect of PHS perinatal HIV recommendations.DESIGN, SETTING, AND PARTICIPANTS: Analysis of nationwide AIDS surveillance data and data from HIV-reporting states through June 1998.MAIN OUTCOME MEASURES: Trends in AIDS by year of diagnosis, incidence rates of AIDS and Pneumocystis carinii pneumonia (PCP) among infants younger than 1 year from US natality data for birth cohorts 1988 to 1996; expected number of infants with AIDS from national serosurvey data; and zidovudine use data from selected HIV-reporting states.RESULTS: Perinatal AIDS cases peaked in 1992 and then declined 67% from 1992 through 1997, including an 80% decline in infants and a 66% decline in children aged 1 to 5 years. Rates of AIDS among infants (per 100000 births) declined 69%, from 8.9 in 1992 to 2.8 in 1996 compared with a 17% decline in births to HIV-infected women from 1992 (n = 6990) to 1995 (n = 5797). Among infants, PCP rates per 100000 declined 67% (from 4.5 in 1992 to 1.5 in 1996), similar to the decline in other AIDS conditions. The percentage of perinatally exposed children born from 1993 through 1997 whose mothers were tested for HIV before giving birth increased from 70% to 94%; the percentage who received zidovudine increased from 7% to 91%.CONCLUSIONS: According to these data, substantial declines in AIDS incidence were temporally associated with an increase in zidovudine use to reduce perinatal HIV transmission, demonstrating substantial success in implementing PHS guidelines. Reductions in the numbers of births and effects of therapy in delaying AIDS do not explain the decline.